Determining the natural distance taken up by an individual relative to a text for the purpose of reading it is very important when preparing ophthalmic lenses that provide their wearer with personalized optical correction and that are particularly adapted to the individual's near vision.
Document JP 2007-097707 discloses a device enabling said distance to be determined, the device comprising firstly a reading medium made in the form of a tablet placed at a distance that is appropriate for reading a text carried by said tablet, and secondly means for measuring the distance of the head relative to the tablet. Those distance-measuring means typically make use of ultrasound.
In that device, the ultrasound emitter or receiver is placed on the individual's eyeglass frame. That presents the drawback of making it impossible to perform the measurement under conditions in which the individual is in a natural posture, i.e. conditions in which the individual is free to take up any desired position relative to the tablet, so as to occupy reading conditions that are the most comfortable for the individual. It will readily be understood that the presence of an ultrasound emitter or receiver on the individual's eyeglass frame necessarily puts a constraint on the individual's posture.
Another solution as disclosed in document JP 2000-325309 consists in placing the ultrasound emitter and receiver on the tablet and in detecting the ultrasound as emitted and reflected on the individual's head. That method presents the drawback of being relatively inaccurate.
In that known device, the measurement axis of the emitter and of the receiver is substantially perpendicular to the plane of the reading portion of the tablet. In order to enable a measurement to be taken, it is therefore necessary to place the tablet in such a manner that the measurement axes of the emitter and of the receiver point towards the individual, and thus that the plane of the reading portion of the tablet is oriented facing the individual. Unfortunately, that configuration does not correspond to a natural reading posture.